PROJECT SUMMARY/ABSTRACT Almost nothing is known about the gut mycobiome in health or disease. Recently, we made the novel observation that peptide YY (PYY), a gut hormone classically recognized for its role in satiety, is abundantly expressed and packaged in discrete dense core granules of Paneth cells (PC) and secreted luminally, suggesting a previously unrecognized role as an antimicrobial peptide (AMP). PYY is remarkably similar in structure and charge distribution to a class of AMPs typified by the amphibian magainin-2 peptide. Unlike magainin-2, PYY is not anti-bacterial; instead PYY specifically targets the virulent (hyphal), but not commensal yeast forms, of gut fungi, specifically Candida albicans. Thus, PYY could play a key role in maintaining fungal commensalism in the healthy gut. Interestingly, Paneth cell dysfunction has been reported in patients with ileal Crohn's disease (iCD), which has been linked to several genetic variants, immune and inflammatory stress, and other factors. Moreover, genetic variants and immune factors can inhibit PYY's bioconversion to a more active, hydrolyzed form. Altogether, these factors can promote fungal virulence that contributes to the pathogenesis of iCD and may explain the observed increases in fungal load and positive anti-Saccharomyces cerevisiae serum antibodies (or ASCA) that are often associated with risk and severity of iCD. Thus, the proposed studies will not only provide key insights into mechanisms and mediators of host-fungal interactions in the gut, but also potentially transform our understanding of Inflammatory Bowel Diseases (IBD) and identify those patients mostly likely to benefit from measures to restore fungal commensalism. This multi-PI proposal involves two investigators with complementary expertise in the study of IBD, host- microbe interactions (Chang, UChicago), and fungal commensalism and virulence (particularly C.albicans) (Noble, UCSF). All clinical studies and sample acquisition will be performed at the University of Chicago and Northwestern Medical Center. The proposal will apply human subjects research, advances in experimental systems and technologies, and multi-omic approaches to address the hypothesis that PC-PYY dysfunction in iCD, from impaired PYY expression, secretion, and/or bioconversion, promotes increased fungal load and virulence, contributing to immune activation, inflammation, and disease. Specific mechanisms involved in the regulation, processing and actions of PC-PYY in health and IBD will be explored using experimental models derived from patients themselves and PYY-null mice. Finally, the ability to reverse fungal pathology to restore states of commensalism will PYY will be examined. Thus, the innovation of this study lies not only in the discovery, but the promise it brings to developing precision medicine tools and approaches for iCD patients and other human disorders caused by fungal pathogens.